It is known, in the field of clinical analyzers, to dispense patient sample onto a dried slide test element using a slide holder that has a metering shoulder that positions a dispensing probe above the slide test element. Such is shown, for example, in U.S. Pat. No. 4,296,070, especially FIGS. 4-6. In such a holder, a slide is fed into the holder only along its two opposite edges, and if the slide is perfectly dimensioned and perfectly flat when inserted, the holder will hold that slide test element at exactly the right vertical distance from the tip of the dispensing probe, during dispensing.
Although such a construction has worked well in the past, it has not accounted for the fact that in a few cases, the dimensional tolerances of the width, or flatness, of the slide test element are such that the slide element ends up being bowed up or down when it is within the holder, ready to receive dispensed liquid. This in turn creates a variable vertical distance between test element and dispensing probe, hereinafter "Z-axis variability". We have discovered that this can be a problem in a) some colorimetric end-point assays, and rate assays, and in b) most immunoassays requiring a wash liquid to be so dispensed. That is, in case a), namely, said some end-point assays, a ring of color will form that has a different density than the rest of the test area. The ring is located at the circumference of the lens formed by the sample contacting the slide test element. Ideally, the ring forms outside the portion of the test area used for reading. However, Z-axis variability can lead, in some cases, to the ring's location falling within, not without, the read area. This, of course, interferes with the precision of the determination of assay concentration.
In case b) regarding washed immunoassays, it is the delivery of the wash liquid by a dispensing probe in the slide holder that is affected by Z-axis variability as described in commonly-owned companion application U.S. Ser. No. 08/393,632, filed on Feb. 24, 1995, entitled, A Method For Washing Immunoassay Elements, by Merrit N. Jacobs et al. It is important, for thorough washing of the center of the wash application, wherever that might be, that the drops of wash be small when contacting the slide test element. However, with many dispensing probes, the size of the drop is controlled by the Z-axis distance as well as the size of the dispensing platform of the tip of the dispenser. If there is Z-axis variability, the initial drop size can easily exceed the nominal small size that is desired, if the Z-axis distance has increased beyond optimum.
For all of the above reasons, it has been a problem prior to this invention that Z-axis variability has not been adequately controlled.